Spectrometer system

ABSTRACT

A wobbling single slit is provided as the entrance slit of a spectrometer which rectilinearly reciprocates at a constant amplitude, such amplitude being selectively adjustable. A control signal is obtained from a circuit that oscillates the slit for controlling the frequency and phase of the ac signal component detected and amplified from the detector after the superposed dc detected signal component is separated from the ac signal component.

The invention described herein was made in the course of work under agrant or award from the Department of Health, Education and Welfare.

This application is a continuation-in-part of application Ser. No.59,456, filed July 30, 1970, now abandoned.

1. Field of the Invention

This invention relates to spectroscopy and more particularly to animproved spectrometer system for obtaining and analyzing spectrometricdata with increased sensitivity and precision of spectral analysis offrom two to four or more orders of magnitude than normally attainable byuse of conventional spectrometer systems.

2. Description of the Prior Art

Various spectrometer systems have been proposed by many others amongwhich are the following: A. Barringer, Paper 67/196, 60th Annual AirPollution Control Association meeting; B. Williams, et al., "MolecularCorrelation Spectrometry". Appl. Opt. 7 (4) 607-16 (1968); C. "Design ofa Feedback circuit for controlling the amplitude of a mechanicaloscillator", Harris, Master's thesis, University of Florida, (1968); D.Giese et al, Appl. Spectroscopy 9, 78-96 (1955); E. Collier et al, J.Appl. Chem. 6 495-510 (1956); F. Pemsler, RSI 28 (4) 274 and 5 (1957);G. Olson et al, Analyt. Chem 32 (3) 370-373 (1960); H. Balslev, Phys.Rev. 143 (2) 636-647 (1966); I. Arumu et al, RSI 37 (12) 1696-1698(1966); J. Bonfiglioli et al, Appl. Opt. 3 (12) 1417-1424 (1965); K.Bonfiglio et al, Appl. Opt. 6 (3) 447-455 (1967); L. Gilgore et al, RSI38, 1535 and 6 (1967); M. Overend et al, Appl. Opt. 6 (3) 457-466(1967); N. Perregaux et al, Appl. Opt. 7 (10) 2031-2035 (1968); and O.Stauffer et al, Appl. Opt. 7 (1) 61-65 (1968).

References A, C, L. N. and O. above describe different techniques andarrangements of moving the radiant energy beam in the spectrometersystem and are in general more directly related to the present inventionthan the other references cited hereinabove.

In principle the present invention relates essentially to an improvedspectrometer system for comparing the intensity of light of a particularwavelength with the intensity of light of wavelengths immediatelyadjacent to that wavelength in the same spectrum. In that sense there isa resemblance of this system to the general art of light "choppers" inwhich two different sources of light either of the same or of differentwavelengths may be compared many times a second as well known in thefield of infrared spectroscopy. Another arrangement is disclosed byBarringer et al in Reference A above. In their system light from acontinuous source is introduced through some known or assumed length Lof a path containing a concentration c of a as which is to be measured.The light is dispersed in the conventional manner in the spectrometer soas to produce an array of more or less overlapping images of theentrance slit in the colors characteristic of the light source asmodified by absorption due to the gas in the amount cL. Their system hasa correlation mask set at the focal plane of the spectrometer, whichmask is an array of slits of spacing corresponding to the absorptionspectrum of the absorbing gas to be investigated. The spectrum isoscillated torsionally to displace the entrance slit image from side toside with respect to the mask, across the point of register. At thepoint of register, the light transmitted through the mask, and thence tothe light detector, has a minimum value of a magnitude that depends onthe product cLμ where μ is a function of the various absorptioncoefficients of the gas at the positions of the slits in the mask. Ifthe frequency of the motion of the spectrum relative to the mask is n,then the signal will be increased at all points out of register, and anac signal will be generated of a frequency pn where p is some wholenumber less than 10. This ac signal can be amplified by use of a tunedamplifier; and the amplified ac signal, when the mean output ismaintained constant by use of AGC, will result in a measure of thequantity cL of the gas. Barringer et al assert that their arrangementhas some degree of specificity in the detection of the gas correspondingto the mask, although the arrangement is designed to detect only one gasat a time, other gases requiring the use of other differently slittedmasks, which normally can only be installed at the factory.

SUMMARY OF THE INVENTION

In accord with various aspects of the invention a wobbling slit means inthe form of a single slit in a movable carrier is provided in theradiant energy path between the radiant energy source and thespectrometer. A photomultiplier detector and an absorption cellcontaining a medium to be investigated communicate with the radiantenergy path between the spectrometer and detector, the detectorincluding a photomultiplier tube having grid wires. The wobbling slit isdisposed in noninterferring relation to the grid wires of the tube.Means are provided for rectilinearly reciprocating the carrier whichincludes circuit means for maintaining the amplitude of thereciprocating motion of frequency constant and selectively adjustablemeans for altering the amplitude. The detector receives the radiantenergy after passage through the absorbing medium to be investigated andconverts the received radiant energy into a signal. A tune phasedlock-in amplifier means receives the signal and converts same into afirst ac signal of selected frequency and a dc signal, the first and dcsignals being coupled to a potentiometric recorder to record the ratioof the first signal to the dc signal. The circuit means also provides acontrol signal representative of the frequency and phase of theoscillating slit motion to the amplifier means for use in rectifying thefirst signal.

The present invention provides an improved spectrometer system in whichthe motion of the spectrum is provided by a single slit wobbling maskset in front of and functioning as the entrance slit of thespectrometer, and the received ac signal with superposed dc signal isseparated and the ac component thereafter amplified in a tuned amplifierand divided by the dc component, thereby rendering the ratio output ofthe system to be independent of the fluctuations in the intensity of theradiant energy source.

The primary advantages of the present invention are that a single slitis used, rather than an array of slits, as the mask; a wobbling systemis employed to move the slit; the amplitude of oscillation of thewobbling system is adjusted at will, and provided to be constant, onceset; the ac signal from the photomultiplier detector is separated fromthe dc signal, and one particular frequency is amplified by use of atuned amplifier, the amplifier output being rectified by use of asynchronous switching circuit, and then divided by the dc component ofthe original photomultiplier detector signal, by use of a potentiometricrecorder whereby the ratio of the recorded output is independent of theintensity and drift of the radiant energy source; the wobbling slit ismounted at the position of the entrance slit of the spectrometer in amanner as to render unaltered the operation of the spectrometer, and topermit use of the spectrometer, after suitable switching adjustments,either as a conventional transmission spectrometer or a derivativespectrometer; and the location of the wobbling slit at the position ofthe entrance slit maintains the light pattern due to the exit slitstationary with relation to the grid wires in the photomultiplier tubeof the detector, which eliminates a large zero signal and instrumentadjustments that otherwise would be produced if the wobbling slit werepositioned as the exit slit of the spectrometer, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features which are believed to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconnection with the accompanying drawing in which:

FIG. 1 is a diagrammatic view of the spectrometer system in accord withthe present invention;

FIG. 2 is a pictorial view, partly broken away, of the wobbling entranceslit for the spectrometer of FIG. 1; and

FIG. 3 is a circuit diagram of the wobbler driver for driving theentrance slit of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more particularly to the drawing of FIG. 1, thespectrometer system is generally indicated by reference numeral 10 andincludes a radiant energy source 11 and a mirror 12 reflecting theradiant energy through a wobbling slit means 13 and into spectrometer 14which may be of the type of a small Jarrell-Ash quater-meter gratinginsturument, model 82-405, marketed under the name "Minichromator",manufactured by Jarrell-Ash Co. of Waltham, Mass. The radiant energypasses through the slit, hereinafter more fully described in conneithFIG. 2, which serves as or functions as the entrance slit of thespectrometer 14. The radiant energy is dispersed by the spectrometer 14prior to passage thereof out the exit slit 15 thereof into theabsorption cell 16 containing a medium to be investigated, such mediumnormally being gaseous with pollutants therein.

The radiant energy not absorbed, exists from cell 16, is detected by adetector 17, and is thereafter analyzed. The detector 17 includes aphotomultiplier tube 18, or the like, which normally includes aplurality of grid wires 19 therein. Since the exit slit 15 of thespectrometer 14 is stationary, the radiant energy pattern is stationarywith respect to the grid wires 19 in the tube 18. Movement or choppingof the radiant energy pattern would otherwise cause radiant energyinterference between the grid wires and a movable exit slit, forexamples, if the wobbling slit were placed as the exit slit of thespectrometer, as disclosed in "Design of a Feedback Circuit forControlling the Amplitude of a Mechanical Oscillator", William T.Harris, Master's Thesis, University of Florida, 1968, which wouldproduce a large zero signal from the photomultiplier tube 18 due to theinterrelation of such wobbling exit slit with respect to thephotomultipler grid wires.

The signal from the detector 17, consisting of a dc signal with the acsignal superposed, is supplied to a phase lock-in amplifier meansindicated at 20 with a coupled potentiometric recorder 21 to make arecord proportional to the ratio of the ac signal to the dc signal. Byappropriate studies of the particular record various characteristics ofthe medium under investigation in the cell 16 may be determined bymethods and means well known to those skilled in the art.

The signal from detector 17 is amplified by ac-dc amplifier 22 with thedc signal component being applied through line 23 to the potentiometricrecorder 21 reference voltage input 24. The ac signal component ispassed through a coupling capacitor 25 to a tuned amplifier 26 which maybe tuned to the same frequency as that of the wobbling slit means,hereinafter more particularly described, or to a harmonic thereof. Ifthe frequency of the tuned amplifier 26 is the fundamental of thefrequency of the wobbling slit means, the system 10 functions as afirst-derivative spectrometer system. However, when the tuned frequencyis twice the frequency of such wobbling slit means, the system 10functions as a second-derivative spectrometer having sensitivities andcapabilities of more accurately identifying, for example, variouspullutants in a sample to be investigated, of from two to four or moreorders of magnitude more sensitivity that that normally achievable.

The tuned ac signal component is fed into a chopper 27, also tuned tothe second harmonic, which rectifies the ac signal component suppliedthereto, chopper 27 being controlled by a control signal from wobblingslit means 13 through a phase shifter 28 to adjust the phase of thecontrol signal and to a frequency doubler 29 and squarer 32 to developthe proper control signal or pulses which activate chopper 27. Theoutput signal from chopper 27 is filtered by filter 30 before suchsignal is coupled to the input 31 of potentiometric recorder 21.

The wobbling slit means is seen to include a carrier 35 carrying asingle slit 36 which functions as the entrance slit of the spectrometer14. The carrier 35 is attached to a support 37, support 37 being rigidlyconnected to a movable member 38. A pair of flat leaf springs 39 areclamped to the spring base 40 as illustrated and are also clamped tomovable member 38 spaced above base 40. Base 40 is rigidly affixed to astationary plate 41 which has an opening (not shown) therethrough forthe passage of the radiant energy from source 11 and mirror 12therethrough and through entrance slit 36 of the spectrometer 14. A rod42 is connected between member 38 and an elongated permanent magnet 43with a pair of solenoids 44 and 45 encircling respective ends 46 and 47of magnet 43.

It is thus seen that wobbling slit means 13 includes a carrier 35 havinga single slit 36 therethrough and means for oscillating such carrier forreciprocating motion, such latter means herein shown as including thepair of leaf springs 39, as well as the other structural components, asdescribed in connection with FIG. 1 together with a wobbler drivercircuit 50 as shown specifically in FIG. 3.

When the wobbler driver circuit 50 is inoperative, the pair of leafpsrings 39 maintain the slit 36 in a null position with the respectivepoles 46 and 47 of the bar magnet 43 being in the center of the solenoidcoils 44 and 45. Upon displacement of the magnet 43 further into eithercoils 44 and 45, the leaf springs 39 tend to retard such displacementand exert restoring forces substantially proportional to thedisplacement.

Assume that solenoid coil 44 is the driving coil and that this coilcauses magnet 43 to be driven further into solenoid pick-up coil 45, themovement of magnet 43 also moving the coupled slit carrier 35. A currentis induced into coil 45 by the movement of the magnet 43 axiallythereinto which essentially is in the form of a sine wave that iscoupled through capacitor 52 to the base 53 of tranistor 54 with properbias voltage being supplied from source 55 through resistors 68 and 69.The transistor 54 is connected essentially as a common-emitter circuit,generally analogous to a grounded-cathode amplifier, which amplifies thesignal applied to base 53 without appreciable distortion. The amplifiedsignal is coupled from collector 57 through capacitor 58 to the base 59of another transistor 60 for power amplification of the signal which isin turn coupled from collector 61 to driving coil 44.

However, the springs 39 have heretofore restored the magnet to its nullposition or slightly beyond with the magnet 43 being within the coil 44to a greater extent than at the null position. The current in the coil44 from collector 61 of transistor 60 again drives the magnet 43 backinto coil 45 and the oscillation and circuit operation continues aspreviously described.

The bias at the base 59 of transistor 60 is variable by employing apotentiometer 65 which is adjusted to chop the signal supplied to thebase 59 either at saturation or cut-off of transistor 60. The currenttransmitted through the collector 61 of transistor 60 into the solenoidcoil 44 is a strong function of the amplitude of the wobbler slit peakto peak displacement, so that when potentiometer 65 is so adjusted andset, the amplitude of the wobbler slit is substantially constant and isunchanging with respect to time. The circuit details of FIG. 3 may bemodified without departing from the spirit or scope of this invention bypersons skilled in the art, it being important to supply a non-linearcircuit element in such wobbler driver 50 to control the amplitude ofoscillation of the wobbler slit.

The spectrometer system as described above does resemble the systems ofBarringer and Harris set forth hereinabove, but the similarities arequite superficial, and the prior art systems do not achieve the resultsobtained by the instant invention nor the many advantages affordedthereby. The most important distinction is the use of a single wobblerentrance slit 13 in the instant system without the use of any type ofcorrelation mask as shown and described in Barringer and Harris.

The single wobbler entrance slit 13 in accord with this inventionrectilinearly reciprocates and causes a sinusoidal variation of thewavelength of the light detected by the photomultiplier tube 18. Lightchoppers used in conventional infrared technology usually compare a pairof independent light sources, which are very much more independent thanthe light of wavelengths immediately proximate in the spectrum, asemployed in the instant system. When two independent sources arecompared by use of the conventional light chopper systems, the observeddifferences in such sources have magnitudes that show a randomdistribution about a means value, the observed differences beingsomewhat more random than the largest fluctuation or variant of eitherof the two sources. However in the spectrometer system in accord withthis invention in using a single wobbler entrance slit 13, thedifferences being measured are the differences in the transmission ofclosely neighboring wavelengths in a given spectrum of the light afterpassage through the absorption cell 16, and the differences aresubstantially independent of fluctuations or variations in the lightsource 11, or of the transmission characteristics of the absorbingmedium through which the beam passes. Any fluctuation or variation inthe intensity of the light source 11, which may be caused by voltagefluctuations, source depletion, etc., or of the transmissioncharacteristics will effect all of the entire wavelengths of light beingobserved at the same time. Thus, the measured difference is free of suchvariations or fluctuations as would be observed if the spectral regionscompared were wholly independent of each other.

The use of a single wobbler entrances slit 13 in accord with theinvention provides at least two important advantages and results notpreviously achievable in either the systems of Barringer or Harris, orin any other prior art known to applicant. First the concentration ofany and all gases having line absorption spectra in the range ofsensitivity of the instant system can be observed and measured withoutthe necessity of changing correlation masks or the like. The observationof any and all spectra is performed by merely slowly rotating thediffraction grating as in the wavelength scan of the conventionalJarrell-Ash quarter-meter grating instrument 14 hereinabove set forth.Thus, the instant system permits observation and recording of theconcentration of a plurality of pollutant gases without any requiredchanging of the correlation masks of the prior art.

Second, the determination of a pollutant gas concentration is strictlylinear function of the signal intensity from the detector of the instantinvention. The output signal from detector 17 is a measure of thecurvature in the spectral intensity curve expressed as a function ofwavelength, and it varies strictly linearly with concentration of theabsorbing gas in the radiant energy path, while the output signal fromthe detector of Barringer or Harris is not a linear function of suchconcentration. When a prior art multi-slit mask is employed, eachdifferent pollutant gas presents a different relation betweenconcentration and signal strength, and when different light sources areused or if different pollutant concentrations are present in the gasbeing analyzed, different relationships between the concentration andsignal strength are encountered. The use of a single wobbling slit inaccord with the instant invention produces a signal strength strictlyproportional to the pollutant concentration throughout the range of thegrating instrument 14. Thus, a single observation of the signal outputfor a known concentration of gas is all that a person needs in theinstant system for calibrating same, for any unknown concentration ofthat gas, or for any light source that may be used in the instantsystem, whether the unknown concentration of that gas is present in theabsorption cell 16 by itself or with any other pollutant gases in thesample being analyzed. While the Barringer, Harris and the instantinvention can all function to determine the presence or absence of aparticular gas (if a plurality of correlation masks are used) theBarringer and Harris system do not provide a measure of theconcentration of such gas without the use of a calibration curveobtained by use of a plurality of calibration points, or even acomparison of a standard sample for any given measurement, since therelationship between the gas concentration and the output signals is notlinearly related; nor is any simple calibration technique known whichwould provide reasonably accurate concentration readings; i.e., withoutusing a plurality of calibration points to construct a calibration curvefor each mask and different gas for each light source used in theBarringer and Harris systems.

An exemplary embodiment of the circuit components of FIG. 3 is seen toinclude the following parameters;

    ______________________________________                                        Solenoid coils 44 and 45 64 mh                                                Capacitors 52 and 58     10 μf                                             66                       5 μf                                              Resistors 56             18.2K ohms                                           65 and 68        100K ohms                                                    67                       2K ohms                                              69                       464K ohms                                            70                       150 ohms                                             Transistors 54           Fairchild 2N3565                                     60                       RCA SK3009                                           Power Souce 55           12 v DC                                              Bar magnet 43            Alnico                                               ______________________________________                                    

In the specific embodiment of the invention constructed and tested inaccord with the invention, the wobbler slit 36 was displacedapproximately 1 millimeter peak to peak with the reciprocation oroscillation frequency being 45 cycles per second. These values are notcrictical but have proven to be effective in the functioning andperformance of the spectrometer system constructed in accord with theinvention herein disclosed.

While the invention has been described with respect to a certainspecific embodiment, it will be appreciated that many modifications andchanges may be made by those skilled in the art without departing fromthe spirit of the invention. It is intended, therefore, by the appendedclaims to cover all such modifications and changes as fall within thetrue spirit and scope of the invention.

What is claimed as new and what it is desired to secure by LettersPatent of the United States is:
 1. In a spectrometer system including aradiant energy source, a scanning spectrometer, only a single entranceslit disposed between said source and said spectrometer, said sourceproducing a focused radiant energy beam passing through said entranceslit and into said spectrometer, said spectrometer having a single fixedexit slit whereby a radiant energy path is established from said sourcethrough said entrance slit and through said spectrometer and out saidexit slit, said system including a photomultiplier detector in saidradiant energy path aft of and spaced from said exit slit, an absorptioncell adapted to contain a medium to be investigated, said cellcommunicating with said radiant energy path between said exit slit andsaid detector, said system having no correlation mask corresponding to amedium to be investigated in said cell, the improvement comprising meansfor movably mounting said entrance slit, said means including means forrectilinearly reciprocating said entrance slit at a constant and rapidfrequency to modulate the wavelength of the radiant energy beam detectedin a sinusoidal manner.
 2. In the system as defined in claim 1 whereinsaid detector includes a photomultiplier tube having grid wires, saidmovable entrance slit being located in the radiant energy path betweensaid source and spectrometer whereby radiant energy interference on saidgrid wire is non-existent.
 3. In the system as defined in claim 1wherein said detector receives the radiant energy after passage throughan absorbing medium to be investigated in said cell and converts thereceived energy into a modulated signal, a tuned phase lock-in amplifiermeans for receiving said signal and converting said signal into a firstsignal of selected frequency and a dc signal, said amplifier means beingtuned to the same frequency as the frequency at which said entrance slitis reciprocating.
 4. In the system as defined in claim 1 wherein saiddetector receives the radiant energy after passage through an absorbingmedium to be investigated in said cell and converts the received energyinto a modulated signal, a tuned phase lock-in amplifier means forreceiving said signal and converting said signal into a first signal ofselected frequency and a dc signal, said amplifier means being tuned totwice the frequency as the frequency at which said entrance slit isreciprocating.
 5. In the system as defined in claim 1 wherein said meansfor rectilinearly reciprocating said entrance slit includes circuitmeans for maintaining constant the amplitude of the reciprocating motionof said entrance slit.
 6. In the system as defined in claim 5 whereinsaid circuit means includes a selectively adjustable means for alteringthe amplitude of the reciprocating motion of said entrance slit.
 7. Inthe system as defined in claim 1 wherein said detector receives theradiant energy after passage through an absorbing medium to beinvestigated in said cell and converts the received radiant energy intoa modulated signal, a potentiometric recorder, a tuned phase lock-inamplifier means for receiving said signal and converting said signalinto a first signal of selected frequency and a dc signal, said firstand dc signals being coupled to said recorder to record the ratio ofsaid first signal to said dc signal.
 8. In the system as defined inclaim 7 wherein said means for rectilinearly reciprocating said entranceslit includes means for providing a control signal representative of thefrequency and phase of the reciprocating motion of said entrance slit tosaid amplifier for use in rectifying said first signal.
 9. In the systemas defined in claim 8 wherein said amplifier means is tuned to the samefrequency at which said entrance slit is reciprocating.
 10. In thesystem as defined in claim 8 wherein said amplifier means is tuned totwice the frequency at which said entrance slit is reciprocating.
 11. Ina spectrometer system including a radiant energy source, a spectrometer,only a single entrance slit disposed between said source and saidspectrometer, said source producing a focused radiant energy beampassing through said entrance slit and into said spectrometer, saidspectrometer having a single fixed exit slit whereby a radiant energypath is established from said source through said entrance slit andthrough said spectrometer and out said exit slit, said system includinga photomultiplier detector, an absorption cell adapted to contain amedium to be investigated, said cell communicating with said radiantenergy path between said exit slit and said detector, said detectorreceiving said radiant energy after passage thereof through said celland producing an output signal, the improvement comprising means forrectiliearly reciprocating said entrance slit at a constant and rapidfrequency to modulate the wavelength of radiant energy beam detected ina sinusoidal manner, a potentiometric recorder for producing a ratio oftwo signals both derived from said output signal of said detectorwhereby said ratio is invariant with changes in the intensity of saidsource.
 12. In a spectrometer system including a radiant energy source,a scanning spectrometer, said source producing a focused energy beampassing into said spectrometer, said system including a photomultiplierdetector, an absorption cell adapted to contain a medium to beinvestigated, means for sinusoidally varying the wavelength of theradiant energy beam from said source into said spectrometer at aconstant and rapid frequency, said means including a singlerectilinearly reciprocating entrance slit between said source and saidspectrometer, said sinusoidally varying beam passing from saidspectrometer through said absorption cell onto said detector, means toreceive the output signal from said detector and provide an indicationof the existence and concentration of the medium to be investigated..Iadd.
 13. In a spectrometer system including a radiant energy sourcefor producing a focused energy beam, a scanning spectrometer having asingle entrance slit for receiving the focused energy beam and a singleexit slit for passing the radiant energy beam from said spectrometer,said system including a single photomultiplier detector, an absorptioncell adapted to contain a medium to be investigated and receiving theradiant energy beam from said spectrometer, said system having nocorrelation mask corresponding to a medium to be investigated in saidcell, means for sinusoidally varying the wavelength of the radiantenergy beam from said source into said spectrometer at a constant andrapid frequency, said sinusoidally varying beam passing from saidspectrometer through said absorption cell onto said detector and, meansto receive the output signal from said detector and provide anindication of the existence and concentration of the medium to beinvestigated..Iaddend..Iadd.
 14. In the system as defined in claim 13wherein said exit slit is fixed in its location in saidsystem..Iaddend..Iadd.
 15. In the system as defined in claim 13 whereinsaid detector includes a photomultiplier tube having grid wires, saidmeans for generally sinusoidally varying the wavelength of the radiantenergy beam being located in the radiant energy path between said sourceand said exit slit of said spectrometer whereby radiant energyinterference on said grid wire is non-existent..Iaddend. .Iadd.
 16. Inthe system as defined in claim 13 wherein said detector receives theradiant energy after passage through an absorbing medium to beinvestigated in said cell and converts the received energy into amodulated signal, a tuned phase lock-in amplifier means for receivingsaid signal and converting said signal into a first signal of selectedfrequency and a dc signal, said amplifier means being tuned to saidfrequency..Iaddend..Iadd.
 17. In the system as defined in claim 13wherein said detector receives the radiant energy after passage throughan absorbing medium to be investigated in said cell and converts thereceived energy into a modulated signal, a tuned phase lock-in amplifiermeans for receiving said signal and converting said signal into a firstsignal of selected frequency and a dc signal, said amplifier means beingtuned to twice said frequency..Iaddend..Iadd.
 18. In the system asdefined in claim 13 wherein said means for generally sinusoidallyvarying the wavelength includes circuit means for maintaining constantthe amplitude of the wavelength variation. .Iaddend..Iadd.
 19. In thesystem as defined in claim 18 wherein said circuit means includes aselectively adjustable means for altering the amplitude of thewavelength variation..Iaddend. .Iadd.
 20. The system as defined in claim13 wherein said detector provides a modulated output signal and whereinsaid means to receive said output signal from said detector and providean indication of the existence and concentration of the medium comprisesa tuned phase lock-in amplifier means for converting said modulatedoutput signal into an ac signal component and a dc signal component andan indicator responsive to the ratio between said components..Iaddend..Iadd.
 21. In the system as defined in claim 20 wherein saidmeans for sinusoidally varying the wavelength includes means forproviding a control signal representative of the frequency and phase ofthe wavelength variation of said amplifier for use in rectifying saidfirst signal..Iaddend..Iadd.
 22. In the system as defined in claim 21wherein said amplifier means is tuned to said frequency..Iaddend..Iadd.23. In the system as defined in claim 21 wherein said amplifier means istuned to twice said frequency..Iaddend.